Cellular telecommunication systems are traditionally split into a radio access network part and a core network part. The access network is responsible for allocating radio resources to user terminals, and for managing these resources, depending on the radio access technology. The core network on the other hand includes functionality which is not dependant on the radio access technology being used.
In early GSM networks, only a single core network was provided. This network facilitated circuit switched calls, generally voice calls. The main component of the circuit switched core network is the Mobile Switching Centre (MSC) which provides exchange-type and mobility functionality in combination with Home Location Register (HLR) and Visitor Location Register (VLR). Later, a second core network was introduced to facilitate packet-switched data communication. This was the packet switched core network, i.e. GPRS core network. The main components of a GPRS core network are the Gateway GPRS Support Node (GGSN) which interconnects the access network to the IP world (e.g. the Internet or an IP backbone) and the Serving GPRS Support Node (SGSN) which is responsible for interconnection with the radio access network.
So-called third generation (3G) networks have introduced a new access network known as the UMTS Terrestrial Access Network (UTRAN) which offers significantly higher data transfer speeds than the GSM access network. The main components of the UTRAN are the node-B, which uses WCDMA as the radio transport technology, and the Radio Network Controller (RNC) which controls the node-B's and is responsible for radio resource management. The RNC interfaces the access network to the circuit switched core network through an MSC server and Media Gateway, and to the packet switched core network through the SGSN.
In current 2G and 3G networks, each node within a core network, e.g. MSC or SGSN, is responsible for a “service area” within the access network. Typically, a service area will comprise a plurality of access network nodes, e.g. BSCs (and BTSs) or RNCs (and node-Bs). When a user terminal roams within a service area it remains registered with the core network node responsible for that service area, regardless of whether or not the terminal crosses cell boundaries within the service area. However, when the terminal crosses a cell boundary that represents an inter-service area boundary, the terminal is transferred to the core network node responsible for the new service area. Each transfer results in core network updates as well as HLR updates.
In an effort to reduce network traffic associated with service area handovers, 3GPP has defined a so-called “Iu-flex” interface between access network and core network nodes, see 3GPP TS 23.236, Intra-domain connection of Radio Access Network (RAN) nodes to multiple Core Network (CN) nodes both in the control plane and in the user plane. This interface provides for the flexible allocation of core network nodes to access network nodes and introduces the concept of “pool-areas”. A pool area is similar to a service area insofar as it encompasses a set of access network nodes (typically within a common geographical area). However, a service area can be associated with more than one core network node, e.g. MSC or SGSN. User terminals are allocated to core network nodes within the same pool area according to some unspecified algorithm. This could be based upon load sharing. Pool areas may be contiguous or may be overlapping to a greater or lesser extent.
As a user moves across a pool area, he remains registered with the same core network node, regardless of his geographical location with respect to that node. Thus, update related traffic within the core network is avoided. When the user crosses a pool area boundary, he is transferred to a core network node within the new pool area.
As well as reducing core network update traffic, the pool area concept introduces a degree of redundancy into the system architecture as, if one core network node fails, users can be dynamically re-assigned to an alternative node. Furthermore, capacity within a pool area can be easily upgraded (or even downgraded) by adding (or taking away) core network nodes.
It is likely that future generations of cellular networks will include the concept of pool areas. For example, 3GPP TR 23.882, System Architecture Evolution: Report on Technical Options and Conclusions, discusses this architecture in the context of the System Architecture Evolution (SAE) network. In theory at least, the larger a pool area is, the more efficient it will be at decreasing core network traffic. However, in the case of large countries, a single operator's network may span several thousand kilometers and a pool area that large can result in users being allocated to extremely remote core network nodes, introducing tens of milliseconds of delay between the core network node and the user. This can be a problem in the control plane, as the path between the user and the core network node might be traversed multiple times in a control procedure, increasing the latency of the control procedures. In the user plane, large distances between the user and a user plane node within the core network could lead to high transport costs. In the absence of any alternative solution, large pool areas will be avoided.